Over charge protection device

ABSTRACT

An over-voltage protection device includes a substrate including an upper surface and a lower surface; a first electrode provided on the upper surface of the substrate; a second electrode provided on the lower surface on the substrate; a first conductive layer overlying the lower surface of the substrate, the first conductive region being a conductive region of a first type; a plurality of first conductive regions provided proximate the upper surface of the substrate, the plurality of first conductive regions being conductive regions of the first type; and a plurality of second conductive region provided proximate the upper surface of the substrate, the plurality of second conductive region being conductive regions of a second type. The plurality of the first conductive regions are provided in an alternating manner with the plurality of second conductive regions. The first electrode is contacting the plurality of the first and second conductive regions.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to U.S. patent application Ser.No. 10/930,276, filed on Aug. 30, 2004, which claims priority to U.S.Provisional Patent Application No. 60/501,963, filed on Sep. 10, 2003,which are both incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor power device, morespecifically, a an over-charge protection device.

In some applications for laser-diodes (e.g. welding) high laser power isnecessary. This is achieved by electrically connecting many laser-diodes(e.g., 10 or more) in series, a solution that requires one power sourceor current source.

With series connection, one requires more acceptable, readily availablevoltage range for driving these laser diodes in series, 10 volts ormore, rather than the low voltage, 1 volt to 3 volts that one such diodewill need. The same argument holds for the series connection of lightemitting diodes, (LED's), for lighting applications.

Another advantage of a series connection is higher power output comparedto higher current in a parallel connection. For same power output in aparallel connection, the current is higher. For example, for 10 diodesin parallel, the current is 10 times higher than in a series connection,thus the conduction losses in the associated conductors are 100 timeshigher (power loss increases by the square law of current). A problemarises, however, when one of the laser-diodes, (or LED's) in aseries-connection fails and is destroyed. This may make the entiredevice unusable although other nine diodes are in good operationalcondition.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an overvoltage protection device thathas the characteristics of a diode and resistor. The overvoltageprotection device is accordingly referred to as “diristor.”

In one embodiment, the diristor includes a substrate of firstconductivity, a layer of second conductivity, a plurality of conductivewells of first type formed proximate an upper surface of the diristor,and a plurality of conductive wells of second type formed proximate theupper surface of the diristor. The conductive well of first type andconductive well of second type are placed in alternating order.

In one embodiment, an over-voltage protection device includes asubstrate including an upper surface and a lower surface; a firstelectrode provided on the upper surface of the substrate; a secondelectrode provided on the lower surface on the substrate; a firstconductive layer overlying the lower surface of the substrate, the firstconductive region being a conductive region of a first type; a pluralityof first conductive regions provided proximate the upper surface of thesubstrate, the plurality of first conductive regions being conductiveregions of the first type; and a plurality of second conductive regionprovided proximate the upper surface of the substrate, the plurality ofsecond conductive region being conductive regions of a second type. Theplurality of the first conductive regions are provided in an alternatingmanner with the plurality of second conductive regions. The firstelectrode is contacting the plurality of the first and second conductiveregions.

In another embodiment, a laser diode module includes a laser diode; anover-voltage protection device in parallel connection to the laserdiode; and an electrical contact coupled to both the laser diode and theover-voltage protection device. The over-voltage protection deviceincludes a substrate including an upper surface and a lower surface; afirst electrode provided on the upper surface of the substrate; a secondelectrode provided on the lower surface on the substrate; a firstconductive layer overlying the lower surface of the substrate, the firstconductive region being a conductive region of a first type; a pluralityof first conductive regions provided proximate the upper surface of thesubstrate, the plurality of first conductive regions being conductiveregions of the first type; and a plurality of second conductive regionprovided proximate the upper surface of the substrate, the plurality ofsecond conductive region being conductive regions of a second type. Theplurality of the first conductive regions are provided in an alternatingmanner with the plurality of second conductive regions. The firstelectrode is contacting the plurality of the first and second conductiveregions.

In yet another embodiment, a semiconductor device to provideover-voltage protection includes a substrate including an upper surfaceand a lower surface; a first electrode provided on the upper surface ofthe substrate; a second electrode provided on the lower surface on thesubstrate; a first conductive layer overlying the lower surface of thesubstrate, the first conductive region being a conductive region of n+type; a second conductive layer the first conductive layer, the secondconductive layer being a conductive region of n+ type; a plurality offirst conductive regions provided proximate the upper surface of thesubstrate, the plurality of first conductive regions being conductiveregions of n+ type; and a plurality of second conductive region providedproximate the upper surface of the substrate, the plurality of secondconductive region being conductive regions of p+ type. The plurality ofthe first conductive regions are provided in an alternating manner withthe plurality of second conductive regions. The first electrode iscontacting the plurality of the first and second conductive regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate cross-sectional views of over-chargeprotection devices according to embodiments of the present invention.

FIG. 2 illustrates a graph a current characteristic of an over-chargeprotection device of one embodiment of the present invention.

FIG. 3 illustrates a cross-sectional view of an over-charge protectiondevice of one embodiment of the present invention.

FIG. 4A illustrates a top view of an over-charge protection device ofone embodiment of the present invention.

FIG. 4B illustrates an over-charge protection device of anotherembodiment of the present invention.

FIG. 5 illustrates a cross-sectional view of a laser diode module.

FIG. 6 illustrates an over-charge protection device configured to beless sensitive to an adjacent light source according to one embodimentof the present invention.

FIG. 7 illustrates an over-charge protection device configured to beless sensitive to an adjacent light source according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For illustrative purposes, the over-voltage protection device ordiristor of the present invention is described in connection with laserdiodes. The diristor, however, may be applied to other types of powerdevices or diodes.

In series-connected laser diodes, if one of them is short-circuited, thecurrent continues to flow through the remaining laser-diodes so thatthey continue to operate. In many cases, however, a failed laser-diodecauses an interruption of the current flow because of a high resistanceor disconnected (molten) leads. An interruption of the current flowthrough one of the laser-diodes then causes the whole stack of seriesconnected devices to stop functioning.

One embodiment of the present invention provides an over-voltageprotection device that can be connected in parallel to a laser-diode (orany other device with similar characteristics). Under normal operatingcondition, the current flow through the laser-diode causes a voltagedrop of typically 1V-3V. In this case the current flowing through theover-voltage protection device should be as small as possible.

Any increase of the voltage drop across the laser-diode above a givenpotential, e.g., above 2V-3V, indicates a failure of the device. In thiscase, the parallel-connected protection device (or diristor) is turnedon and conducts the current with a low voltage drop, which preferably isthe same as the normal voltage drop of a functioning laser-diode. Asemiconductor device described below has one or more of the abovefeatures.

In FIG. 1A, a device or diristor includes a silicon substrate or chipwith an n− region 1 (or a first conductive region of first conductivetype) having a low doping concentration and an n+ region 2 (or a secondconductive region of first conductive type) with a high n+ type dopingconcentration. The n+region is formed over the bottom surface, and then− region is formed on the n+ region. The top surface is patterned withalternating n+ regions 3 (or a third conductive region of firstconductive type) and p+ regions 4 (or a fourth conductive type of secondconductive type). The bottom surface is covered with a metal layer 7forming the cathode contact, and the top surface is covered with a metallayer 6 forming the anode contact. The anode is in contact with both n+and p+ regions 3 and 4.

FIG. 1B illustrates a diristor having n+ regions 3′ and p+ regions 4′contacting each other in alternating manner. A region 5 illustrates adiffused region that includes dopants from both the n+ region and p+region.

FIGS. 4A and 4B illustrates top views of diristors according toembodiments of the present invention. The pattern of n+ and p+ regions 3and 4 can be for example p+ islands that are surrounded by an n+ area(see FIG. 4A), or n+ islands that are surrounded by a p+ area, oralternating stripes of different conductivity type (see FIG. 4B). FIG.1B is a cross-sectional view of the device taken along arrows A and A′in FIG. 4A or 4B.

When a small positive voltage is applied to the anode contact to provideforward bias, an electron-current flows from n+ regions 3 to n+ region 2via n− region 1. Under such a condition, the diristor behaves like anohmic resistor. The current increases approximately linearly with theapplied voltage. The value of the resistance is determined by thespecific resistivity and thickness of the n− region 1, as well as by thesize and geometry of the n+ regions 3 and the total area of the chip. Inorder to obtain a desirable low current in this mode of operation, thespecific resistivity of n− region 1 can be chosen high, (higher than 5ohm-cm), for example 70 ohm cm. The current is kept low during thisstate to minimize the current from being drained away from the laserdiode to which the diristor is in parallel connection.

The current flowing from n+-regions 3 through the n− region 1 is spreadlaterally under the p+ regions 4 and thereby causes a lateral voltagedrop. If the current exceeds a certain limit, this lateral voltage dropcauses a forward bias on the pn-junction between regions 4 and 1, sothat holes are injected from p+ regions 4 into the n− region 1. Aresulting hole-current increases the carrier concentration in the n−region 1, so that the voltage drop between the anode and cathodedecreases.

As a result, the diristor has the current-voltage characteristics of aresistor with small currents increasing linearly at low forward voltages(i.e., below a given threshold voltage), and the low voltage drop of aforward-biased diode once the applied voltage has exceeded the givenvoltage (see FIG. 2).

In FIG. 2, the diristor functions like a resistor as the voltage dropincreases to a snapback voltage Va, e.g., 3 volts. Once the appliedvoltage has exceeded the snapback voltage Va at point “a”, the currentcan be increased to very high levels with a small voltage drop betweenthe anode and cathode, like a diode with a conduction voltage Vf asshown as point “b”. The snapback voltage is the threshold voltage atwhich point the diristor converts its characteristics from aresistor-like behavior to a diode-like behavior.

The characteristics of the diristor can be tailored to provide a desiredsnapback voltage Va and conduction voltage Vf based on process anddesign parameters. Exemplary process/design parameters are illustratedbelow: (i) a structure like in FIG. 1A or 1B can be used, where the n+region 3 is separated from the p+ region 4, with equal separation acrossthe die, or varied separation distance across the die; (ii) the depth ofthe n+ region 3 can be equal or different than the depth of the p+region 4; (iii) the relative size, in surface area and volume of the n+region 3 can be equal or different than the size of the p+ region 4;(iv) the doping concentration of the n+ region 3 can be equal ordifferent from the doping of the p+ region 4.

In one embodiment, some of the interesting performance parameters of thediristors are the current level at which it snaps back Ia, as shown inFIG. 2, and the snapback voltage Va. As explained above, thecharacteristics show a resistor type behavior, in voltage from 0 voltsto Va. The value of the resistance (Va/Ia) is dependant on theresistivity of the n− region 1, and also by the relative area of the n+region 3 (also n+ regions 13 and 23 in FIG. 3). The doping or carrierconcentration in the n+ regions also affect the resistance. The voltageof snapback Va is affected by the geometrical parameters of thecorresponding p+ regions 4 (also p+ regions 14 and 24 in FIG. 3). Thedoping or carrier concentration in the p+ regions affect the Va. Forexample, the snapback voltage may be decreased by increasing the size ofthe p+ region 4 or increasing the resistivity of the n− region 1. Thediristor characteristic's reverts back to resembling a resistor if thecurrent flow is reduced below the current level “b”.

By practicing the variations described above, the snapback voltage Va inthe range of 1-3 volts can be produced with a snapback current Ia of 1miliamperes to 1 ampere. For other applications, it is possible toprovide a diristor having a higher snapback Va than the 3 volts and ahigher snapback current than 1 ampere.

When the polarity of the applied voltage is reversed (negative on theanode), an electron-current flows from n+ region 2 to n+ regions 3.Again the device behaves like an ohmic resistor with the same value ofthe resistance. However, with this polarity of voltage the lateralcurrent flow under the p+ regions 4 causes a reverse bias on thepn-junction between regions 4 and 1 so that no hole-current can beinjected from p+ regions 4 into the n− region 1. Consequently, withnegative anode-voltage the current through the diristor increaseslinearly without the snapback characteristic illustrated in FIG. 2.

FIG. 3 illustrates a diristor that provides over-voltage protection forboth positive and negative polarities of applied voltage. The topsurfaces are patterned with alternating n+ regions 13 and p+ regions 14,and the bottom surfaces are patterned with alternating n+ regions 23 andp+ regions 24. That is, FIG. 3 illustrates a double-sided diristoraccording to one embodiment of the present invention.

One can vary the sizes of the n+ and p+ region on the back side of thediristor, such that the n+ region 13 on the top is equal or differentthan the n+ region 23. Similarly, the size of the p+ region 14 on thetop may be varied relative to the p+ region 24 on the backside. The n+region 13 on the top can also be aligned vertically above the n+ region23 on the bottom. or for different performance align vertically abovethe p+ region 24 on the bottom.

For different characteristics, the double-sided diristor can have thetop side structure of the n+ and p+ regions look like the top side ofthe diristor in FIG. 1A (i.e., like the n+ and p+ regions 3 and 4), andthe backside look like the top structure of the diristor in FIG. 1B(i.e., like the n+ and p+ regions 3′ and 4′), and vice versa, therebycreating a non-symmetrical diristor. Such a diristor providesnon-symmetrical performance characteristics for forward bias andnegative bias.

FIG. 5 illustrates a laser diode module 50 having a laser diode 52 and adiristor 54 provided in parallel connection. The laser diode 52 anddiristor 54 are provided on the same substrate (heat sink) 56 next toeach other with a small distance provided therebetween. The substrate 56may be a copper heatsink, which also serves as electrical connection. Alid 58 couples both the diode and diristor on the top.

The laser diode emits radiation to along a given direction. The backside of a laser diode is usually covered with a mirror (not shown),which reflects the radiation and direct it to the front side of thelaser diode. However, the mirror does not perfectly reflect the light orradiation to the front side and part of the radiation (as indicated bynumerals 60 and 62) may be emitted towards the sides of the laser diode,including the side where the diristor is provided. Part of the light 60can penetrate the outer surface of the diristor at the edge of the diethat is adjacent to the back side of the laser. The light is thenabsorbed inside the diristor and creates electron-hole pairs.

These unintended electron-hole pairs may lower the predefined snapbackvoltage Va. That is, the effective snapback voltage may be less than thepredefined snapback voltage Va due to the increased carriers in thediristor due to the electron-hole pairs generated by the light 60 fromthe laser diode. Similarly, the snapback current Ia may also be lowered.

As a result, the diristor may switch its characteristics from a resistorto a diode at an effective snapback voltage that is too low. In such acase, the diristor would take over the current from the laser diodealthough the laser diode is functioning normally.

FIG. 6 illustrates a diristor 70 that is configured to be less sensitiveto the light 60 received from the adjacent device (e.g., laser diode)according to one embodiment of the present invention. The active area ofthe diristor, which is in contact with the anode 6, is spaced apart froman edge of the die or diristor. For example, a distance w is providedbetween the edge of the die and the anode. This distance w is largerthan the diffusion length of holes in the n− region 1. That is, theseholes generated by the light are recombined before they travel beyondthe distance w. In one embodiment, the distance w is 1, 2 or 3-times thevertical thickness of the n− layer 1.

Generally, light that penetrates a silicon surface, such as a diristor,is absorbed within a certain depth inside the silicon. For light with awavelength of 800 nm, for example, the absorption length—defined as thelength where the intensity drops by a factor of 1/e=0.368—isapproximately 10 μm. The majority of the incident light is absorbedwithin 2 to 3 absorption lengths, equivalent to approximately 30 μm. Theoptically generated holes, however, can diffuse much deeper into the n−layer 1 before they recombine with electrons. The diffusionlength—defined as the length where the hole concentration drops by afactor of 1/e=0.368—can be for example 100 μm or even several 100 μm.

FIG. 7 illustrates a diristor 80 that is configured to be less sensitiveto the light received from an adjacent device according to anotherembodiment of the present invention. The active area of the diristorthat is in contact with the anode 6 is provided a distance w from theedge of the diristor as in FIG. 6. In addition, an area between theanode 6 and the edge of the die is at least partly covered with anelectrode 38, which is electrically separated from the anode. Theelectrode 38 contacts n+ and p+ regions 33 and 34. The electrode 38 canbe floating, i.e. not connected with the external circuit, or it can beconnected to the cathode 5 through external circuitry (not shown).

The electrode 38 facilitates the recombination of the holes generated bythe light 60 of the laser diode. That is, optically generated holesdiffuses into the p+ regions 34, where they are attracted by thebuilt-in electric field at the pn-junction. Similarly, the excesselectrons flow into the n+ regions 33. The electrode contacts the n+ andp+ regions 33 and 34, so that electrons and holes can recombine throughthis contact.

The present invention has been illustrated using specific embodiments.The embodiments illustrated above may be amended, modified, or alteredwithout departing from the scope of the present invention. For example,the structures provided for the diristors 70 and 80 may be provided onthe top and bottom surfaces of a double-sided diristor. The scope of thepresent invention shall be determined based on appended claims.

1. An over-voltage protection device, comprising: a substrate includingan upper surface and a lower surface; a first electrode provided on theupper surface of the substrate; a second electrode provided on the lowersurface on the substrate; a first conductive layer overlying the lowersurface of the substrate, the first conductive layer being a conductivelayer of a first type; a second conductive layer provided over the firstconductive layer, the second conductive layer having a higherresistivity than the first conductive layer; a plurality of firstconductive regions provided proximate the upper surface of thesubstrate, the plurality of first conductive regions being conductiveregions of the first type; and a plurality of second conductive regionsprovided proximate the upper surface of the substrate, the plurality ofsecond conductive regions being conductive regions of a second type, thesecond conductive regions being configured to provide a forward biaswith the second conductive layer, wherein the plurality of the firstconductive regions are provided in an alternating manner with theplurality of second conductive regions, wherein the first electrode iscontacting the plurality of the first and second conductive regions,wherein the over-voltage protection device is configured to act as aresistor when the first electrode is applied with a voltage that is lessthan a snapback voltage and as a forward-biased diode if the voltageapplied exceeds the snapback voltage.
 2. The device of claim 1, whereinthe first conductive layer is n+ layer and the second conductive layeris n− layer, the device further comprising: a plurality of thirdconductive regions provided proximate the lower surface of thesubstrate, the plurality of third conductive regions being conductiveregions of the first type; and a plurality of fourth conductive regionsprovided proximate the lower surface of the substrate, the plurality offourth conductive regions being conductive regions of a second type,wherein the plurality of third conductive regions are provided in analternating manner with the plurality of fourth conductive regions. 3.The device of claim 2, wherein the plurality of the first conductiveregions are n+ regions and the plurality of the second conductiveregions are p+ regions.
 4. The device of claim 1, wherein the firstelectrode is provided a distance w from an edge of the substrate, thedistance w being larger than a diffusion length of holes in the secondconductive layer.
 5. The device of claim 4, further comprising: a thirdelectrode provided between the edge of the substrate and the firstelectrode and on the upper surface of the substrate.
 6. The device ofclaim 5, wherein the third electrode is a floating electrode.
 7. Thedevice of claim 5, wherein the third electrode is electrically coupledto the second electrode.
 8. The device of claim 1, wherein acharacteristic of the device changes according to a potential appliedbetween the first and second electrodes, the device having acharacteristic of a resistor when the potential between the first andsecond electrodes is no more than a first potential, the deviceproviding a characteristic of a diode once the potential between thefirst and second electrodes exceeds the first potential.
 9. The deviceof claim 1, further comprising: a plurality of third conductive regionsprovided proximate the lower surface of the substrate, the plurality ofthird conductive regions being conductive regions of the first type; anda plurality of fourth conductive regions provided proximate the lowersurface of the substrate, the plurality of fourth conductive regionsbeing conductive regions of the second type, wherein the third andfourth conductive regions are arranged in an alternating manner.
 10. Thedevice of claim 1, wherein the device is configured to provide anelectrical current between the first and second electrodes when apotential difference is provided between the first and secondelectrodes.
 11. A laser diode module, comprising: a laser diode; anover-voltage protection device in parallel connection to the laserdiode, the over-voltage protection device; and an electrical contactcoupled to both the laser diode and the over-voltage protection device.12. The module of claim 11, wherein the over-voltage protection deviceincludes: a substrate including an upper surface and a lower surface; afirst electrode provided on the upper surface of the substrate; a secondelectrode provided on the lower surface on the substrate; a firstconductive layer overlying the lower surface of the substrate, the firstconductive layer being a conductive layer of a first type; a pluralityof first conductive regions provided proximate the upper surface of thesubstrate, the plurality of first conductive regions being conductiveregions of the first type; and a plurality of second conductive regionprovided proximate the upper surface of the substrate, the plurality ofsecond conductive region being conductive regions of a second type,wherein the plurality of the first conductive regions are provided in analternating manner with the plurality of second conductive regions,wherein the first electrode is contacting the plurality of the first andsecond conductive regions.
 13. The module of claim 11, wherein theover-voltage protection device further comprises: a second conductivelayer provided over the first conductive layer, wherein the firstconductive layer is n+ layer and the second conductive layer is n−layer.
 14. The device of claim 13, wherein the plurality of the firstconductive regions are n+ regions and the plurality of the secondconductive regions are p+ regions.
 15. A semiconductor device to provideover-voltage protection, the device comprising: a substrate including anupper surface and a lower surface; a first electrode provided on theupper surface of the substrate; a second electrode provided on the lowersurface on the substrate; a first conductive layer overlying the lowersurface of the substrate, the first conductive layer being a conductivelayer of n+ type; a second conductive layer overlying the firstconductive layer, the second conductive layer being a conductive layerof n− type; a plurality of first conductive regions provided proximatethe upper surface of the substrate, the plurality of first conductiveregions being conductive regions of n+ type; and a plurality of secondconductive region provided proximate the upper surface of the substrate,the plurality of second conductive region being conductive regions of p+type, wherein the plurality of the first conductive regions are providedin an alternating manner with the plurality of second conductiveregions, wherein the first electrode is contacting the plurality of thefirst and second conductive regions, and wherein the first electrode isprovide a distance w from an edge of the substrate.
 16. The device ofclaim 15, wherein the distance w is longer than a diffusion length ofholes in the second conductive layer.
 17. The device of claim 16,further comprising: a third electrode provided between the edge of thesubstrate and the first electrode and on the upper surface of thesubstrate.